Safety (S) is a position in American and Canadian football, played by a member of the defense. The safeties are defensive backs who line up from ten to fifteen yards behind the line of scrimmage. There are two variations of the position in a typical formation, the free safety (FS) and the strong safety (SS). Their duties depend on the defensive scheme. The defensive responsibilities of the safety and cornerback usually involve pass coverage towards the middle and sidelines of the field, respectively.

Safeties are the last line of defense, and are thus expected to be sure tacklers. As professional and college football have become more focused on the passing game, safeties have become more involved in covering the eligible pass receivers.[1]

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The strong safety tends to be somewhat larger and stronger than the free safety. However, the word strong is used because he is assigned to cover the "strong side" of the offense, the side on which the big, powerful tight end lines up on offensive plays. The strong safety tends to play closer to the line and assist in stopping the run. He may also be responsible for covering a player, such as a running back or fullback or H-back, who comes out of the backfield to receive a pass. A strong safety's duties are a hybrid of those belonging to a linebacker and those of the other defensive backs, in that he both covers the pass and stops the run. Two of the most notable retired strong safeties are Ken Houston and John Lynch. Among the best active strong safeties are Troy Polamalu and Adrian Wilson.

The free safety tends to be smaller and faster than the strong safety. His job tends to be to keep some distance from the line of scrimmage, watch the play unfold, and follow the ball. The free safety would correspond to the quarterback in man coverage, but as the quarterback usually remains in the pocket the free safety is "free" to double cover another player. On pass plays, the free safety is expected to assist the cornerback on his side and to close the distance to the receiver by the time the ball reaches him. Offenses tend to use the play-action pass specifically to make the free safety expect a run play, which would draw him closer to the line of scrimmage, and reduce his effectiveness as a pass defender. Furthermore, quarterbacks often use a technique to "look off" a free safety, by purposely looking to the other side of the field during a pass play, with the intention to lure the free safety away from the intended target receiver on the other side of the field. This phenomenon often tests how effective a free safety's wit and athleticism is at defending long pass plays. If the offense puts a receiver in the slot, then the free safety may be called upon to cover that receiver. Free safeties occasionally blitz as well. When this happens, the pressure on the quarterback is often very severe since a blitz by a defensive back is not usually anticipated. Free safeties, because of their speed and deep coverage, are often prone to catching interceptions.[2] Standout retired free safeties include Paul Krause, Larry Wilson, and Willie Wood.
Ed Reed and Nick Collins are the most notable currently active free safeties.

Instead of the safeties dividing up their jobs in terms of run support and pass support, the safeties will sometimes divide up the field into a left half and a right half, with each being responsible for anything that comes into his half of the field. This type of division of responsibility is becoming increasingly common,[3] and is called a "cover-2" defense. The cover-2 was first used by the Pittsburgh Steelers in the 1970s, but was made famous by the Tampa Bay Buccaneers of the late 1990s. Led by head coach Tony Dungy and defensive coordinator Monte Kiffin, the Buccaneers built a dominating defense, with strong safety John Lynch at the forefront. This particular variation of the cover-2 is referred to as the "Tampa 2". Since then, the popularity of the cover-2 has soared. Both of the teams in Super Bowl XLI, the Colts and the Bears (the former being coached by Dungy, the latter by his former protégé at Tampa, Lovie Smith), ran a base cover-two defense.